/Engine/Source/Runtime/GeometryCore/Public/Generators/SweepGenerator.h
Utility function for calculating the contribution of triangle normals to average normals. AbNormalized (ie, the normalized vector from the first vertex to the second) is taken as a parameter so that it can be reused in dealing with a planar quad.
void AdjustNormalsForTriangle ( int32 TriIndex, int32 FirstIndex, int32 SecondIndex, int32 ThirdIndex, [TArray](API\Runtime\Core\Containers\TArray)< FVector3d > & WeightedNormals, const FVector3d & AbNormalized )